The present invention relates to the protection of a multi-terminal electric system based on current differential principles and more particularly to a synchronizing system for digital quantities obtained from currents through respective terminals of an associated multi-terminal transmission line.
The establishment of transmission lines has the tendency to include a multitude of terminals, for example, three or four terminals due to such problems as the difficulty of securing their sites, the preservation of the environment etc. This has resulted in the demand that associated protective relay systems be improved in both performance and reliability. Multi-terminal systems have encountered the problems such as internal faults attended with a charging current developed in the protective section thereof and an efflux from the latter, the passage of a power flow through the protective section thereof etc. Thus the multi-terminal systems have been difficult to be protected by the prior art such as the phase comparison relay system.
In order to avoid this difficulty, there have been alreadily proposed current differential protective systems employing the vectorial sum of currents through all terminals of an associated multi-terminal transmission line. Those current differential protective systems have many advantages in view of the protection. For example, those systems utilize the simple principles based on the first Kirchhoffs' law and are high in ability to sense faults. Further they are free from any malfunction such as a power disturbance and the pulling-out and unrequired to compromise their sensitivity with their timings. Therefore the current differential protective systems are suitable for protecting multi-terminal systems.
In order to materialize current differential protective systems as described above, there have been previously proposed simultaneous sampling systems having the digital technique introduced thereinto. One example of those current differential protective systems has fundamentally comprised means for sampling and digitalizing a current flowing through each of two terminals of protected section of a transmission line involved, a transmitter, a receiver and a processing unit disposed at each of those terminals. At each terminal current data thus formed are transmitted to the receiver at the other terminal and the processing unit has compared the current data with similar data resulting from the other terminal. When a fault has occurred within the protected section as determined thereby, the processing unit is operated to trip an associated circuit breaker to disconnect the protected section from the transmission line.
In conventional current differential protective systems such as described above current have been required to be simultaneously sampled at all terminals resulting in a complicated synchronizing circuitry. In addition, as the entire system is operated in time synchronized state, there have been fears that the occurrence of an abnormality on the synchronizing circuitry disables not only the function of protecting the entire system but also all the functions including measurements, adjustments, operation etc. employing data in common with the protective function. Further the non-accommodation to the enlargement of equipments has become remarkable with an increase in number of substations connected to a transmission line involved.
Accordingly it is an object of the present invention to provide a new and improved multi-terminal protective relay system operative on the basis of current differential principles with a simple construction and with an accuracy capable of being sufficiently put to practical use.